This invention relates to the scavenging of oxygen and particularly, although not exclusively, relates to the scavenging of oxygen in containers, for example food or beverage containers.
Polymers such as poly(ethylene terephthalate) (PET) are versatile materials that enjoy widespread use for fibres, films and three-dimensional structures. A particularly important application for polymers is for containers, especially for food and beverages. This application has seen enormous growth over the last 20 years, and continues to enjoy increasing popularity. Despite this growth, polymers have some fundamental limitations that restrict their applicability. One such limitation is that all polymers exhibit some degree of permeability to oxygen. The ability of oxygen to permeate through polymers such as PET into the interior of the container is a significant issue, particularly for foods and beverages that are degraded by the presence of even small amounts of oxygen. For the purpose of this disclosure, permeable means diffusion of small molecules through a polymeric matrix by migrating past individual polymer chains, and is distinct from leakage, which is transport through macroscopic or microscopic holes in a container structure.
Besides food and beverages, other products affected by oxygen include many drugs and pharmaceuticals, as well as a number of chemicals and even electronics. In order to package these oxygen-sensitive products, brand owners have historically relied on the use of glass or metal packaging. More recently, brand owners have begun to package their products in plastic packages which incorporate passive barriers to oxygen and/or oxygen scavengers. Generally, greater success has been achieved utilizing oxygen scavengers; however, oxygen scavenging materials heretofore have suffered from a number of issues. In particular, oxygen scavengers utilized to date rely on the incorporation of an oxidizable solid material into the package. Technologies utilized include oxidation of iron (incorporated either in sachets or in the container sidewall), oxidation of sodium bisulfite, or oxidation of an oxidizable polymer (particularly poly(butadiene) or m-xylylenediamine adipamide). All of these technologies suffer from slow rates of reaction, limited capacity, limited ability to trigger the scavenging reaction at the time of filling the container, haze formation in the package sidewall, and/or discoloration of the packaging material. These problems have limited the use of oxygen scavengers in general, and are especially significant for transparent plastic packaging (such as PET) and/or where recycling of the plastic is considered important.
Co-pending publication number WO2008/090354A1 discloses a container comprising an active substance which is incorporated in the container and is arranged to react with moisture in the container to release molecular hydrogen. The document describes a wide range of potential active substances including metals and/or hydrides. Potential hydrides are stated to be inorganic for example comprising a metal hydride or borohydride or they may be organic. In addition, the active substance could comprise a polymeric matrix, for example a polymeric silicon hydride. The document also states that the active material may be embedded in a polymeric matrix at a preferred level of 4-8 wt % of active substance. The specific examples in the document focus on use of sodium borohydride as the active material.
One problem associated with use of sodium borohydride is that its initial rate of production of hydrogen can be quite low. This may be disadvantageous since it would be preferable for initial hydrogen production to be at an enhanced rate thereby to scavenge headspace oxygen present in a container after initial filling of the container. The rate may then fall off and be sufficient to scavenge oxygen passing through walls of the container.
It has also been found that active substances described in WO2008/090354A, especially sodium borohydride, can react with aldehydes which are important flavour components of foods and beverages. An increased loss of these flavour components by reaction with the active substance may have a detrimental effect on the flavour of the food or beverage—i.e. the flavour is scalped and such scalping may get worse over time. In addition, boron compounds, unless suitably restrained, may migrate into foods or beverages. Such migration needs to be monitored in order to ensure compliance with any applicable regulatory limits applicable to boron compounds. Furthermore, it has been found that it may be difficult to incorporate some active substances such as sodium borohydride with certain polymers due to a reaction between the borohydride and the polymer.
It is an object of the present invention to address the above described problems.